The present invention involves improvements in fiber glass mats that are used as reinforcements in thermoplastic composite materials.
Composites of fiber glass reinforcements and thermoplastic matrix materials can be formed by various molding techniques such as compression molding or stamping. The resulting composites can be used in a wide variety of products where a combination of strength and light weight are desired.
The configuration and type of reinforcement significantly effect the physical characteristics of the composite, such as tensile strength, flexural strength, and impact performance. Preferred reinforcement configurations may vary considerably for different molding processes and molding conditions. One known method of varying the physical configuration of a mat is by needling. Needling has several effects on a mat that are beneficial for high flow thermoplastic laminating applications. These include consolidating the mat to enhance mat strength, modifying loft, and rupturing and opening fiber glass bundles to individual filaments. The rupturing and opening functions improve the ability of the glass fiber reinforcement to flow along with the polymeric matrix material when the laminate is molded. In other words, the fiber glass reinforcement has sufficient mobility to be displaced into features of the molded product during the molding process. This, in turn, improves the appearance of the molded composite products. However, known needling techniques are not able to achieve optimization of all of these factors in a single mat. Needling from only one side of a mat is good for strengthening the mat, but it is difficult to achieve sufficient opening of the mat for improved flow without over-punching the mat and sacrificing strength.
Needling a fiber glass mat is known to produce spikes of fibers protruding from the original surface of the mat. U.S. Pat. No. 4,335,176 (Baumann) discloses needling a fiber glass mat through its thickness from one side whereby one side of the mat has more fiber spikes than the other side. The differential is produced by the orientation of barbs on the needles. Because the mat is needled from one side, the number of needle punches is necessarily uniform throughout the mat. The patent discloses assembling two of these mats for lamination.
U.S. Pat. No. 4,885,205 (Wahl et al.) discloses symmetrically needling a mat from both sides so as to reduce needle penetration depth. The objective is to improve appearance of the laminate by reducing the height of the fiber spikes above the original mat surface due to the needling. The emphasis of the disclosure is on making the needling symmetrical on both sides, both in needle penetration depth and needle penetration density. While some improvements are possible with this approach, it has been found difficult to balance tensile strength with good appearance of the molded product using mats made by the method of this patent.
U.S. Pat. No. 5,580,646 (Jansz et al.) discloses needling a mat on both sides, wherein the needling depth or needle type differs from one side to the other. Needle punch density is the same on both sides. The asymmetric mats that are produced are intended to be laminated in pairs. When high flow properties are produced by this method, it has been found that loft is higher than desirable for some applications.
It would be desirable if certain properties of a fiber glass mat could be enhanced for improved thermoplastic composite molding performance with less compromise in other desirable properties as is incurred with prior art approaches.